Change screenshot function to allow reading from other render targets as well. This is not yet fully implemented, just an API preparation for further features. Most drivers don't support the extra targets anyway. But it will be also possible to ask for different color formats, which should be possible on all drivers. Passing ECF_UNKNOWN will choose an optimal color format without the need of pixel value adaptions.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3744 dfc29bdd-3216-0410-991c-e03cc46cb475